1. Field of the Invention
The present invention relates to a method for injection-molding an article having a hollow portion, and more specifically, a method for injection-molding an article having a hollow portion according to a kind of two-color molding method, and an injection-molding apparatus suiTable for carrying out the above method.
2. Description of Related Art
Conventionally, an article having a hollow portion, in which the article is required not only to have surface properties such as an excellent appearance, slidability and chemical resistance but also to have rigidity, is produced by the following method. That is, (1) a method in which a first member for covering the surface properties (generally, an article having a solid portion) and a second member for covering the rigidity (generally, an article having a hollow portion) are separately injection-molded and then assembled, (2) a method in which a first member for covering the surface properties (generally, an article having a solid portion) is prepared in advance, the first member is inserted into a mold and a second member for covering the rigidity (generally, an article having a hollow portion) is integrally injection-molded, and (3) a sandwich gas injection method.
JP-A-10-281139 (Japanese Patent Application No. 9-90679) disclose a method of producing a paper feeding roller, in which a soft material that comes to be relatively soft after being solidified is injected into a cavity. Then, a resin material that comes to be relatively hard after being solidified is injected into the soft material, and then a gas is introduced into the resin material to break a layer of the soft material with the resin material and press the soft material to a wall surface of the cavity with the resin material.
The above method (1) requires an assembly step, and an article produced by the assembly generally has a problem in that the strength thereof is poor as compared with a molded article which is integrally formed. In the above method (2), for producing (molding) the first member and the second member, it is required to carry out injection molding twice or it is required to prepare the first member beforehand, so that the operation is complicated and that an additional production cost is required. In the above method (3), it is difficult to control the thickness of the portion covering the surface properties, and it is also difficult to constitute a desired portion of a molded article from a material (resin) having excellent surface properties.
The method of producing a paper feeding roller, disclosed in JP-A-10-281139, is basically a combination technology of a sandwich molding method and a gas injection method, and the roller has a structure in which the relatively soft material has a hollow portion and covers nearly the entire region of the roller itself and a support shaft made of the relatively hard resin material. In the paper feeding roller, however, the support shaft is desirably hard, and it is difficult to constitute a desired portion from a desired material in the production method disclosed in JP-A-10-281139. Further, in the sandwich molding method, it is difficult to control the thickness of the soft material, and it is difficult to form the soft material on a surface of the resin material with positional accuracy and thickness accuracy.